The present study is intended to evaluate the usefulness of the measurement of serum thyroglobulin in the patients with a potential differentiated malignant tumor of the thyroid gland. The study is also designed to establish whether serum thyroglobulin measurements can be used to evaluate the recurrence of tumor tissue (metastases) after the original malignant tumor was removed surgically. These data would allow optimalization of the care of these patients with reduction in complexity of the testing and costs of the tests involved. BIBLIOGRAPHIC REFERENCES: Shimaoka, K., A.J. Van Herle, and A. Dindogru. Thyrotoxicosis secondary to involvement of the thyroid with malignant lymphoma. J. Clin. Endocrinol. Metab. 43:64, 1976. Van Herle, A.J., I.J. Chopra, J.M. Hershman and R.W. Hornabrook. Serum thyroglobulin inhabitants of an endemic goiter region of New Guinea. J. Clin. Endocrinol. Metab. 43:512, 1976.